Recently, the technology of multi-level output becomes popular in the field of medium-high voltage high power frequency conversion. A multi-level inverter can output a voltage waveform having a low voltage change ratio because of a large number of output voltage steps. Also, the harmonic wave of the output voltage is reduced with the increasing number of the output levels. Furthermore, the multi-level inverter technology has good performance on reducing the switching loss and conduction loss of a system and reducing the withstand voltage of a transistor and the electromagnetic interference of a system. Therefore, multi-level inverters are employed commonly at present.
In the conventional technology, a typical diode clamping multi-level inverter outputting three levels or more is implemented by connecting in parallel multiple power capacitors connected in series to a direct-current side of a photovoltaic inverter, as shown in FIG. 1. Various switch transistors in the inverter are connected to respective power capacitors through power diodes respectively. Since the power capacitors output unequal power, the voltages across the power capacitors are unequal, i.e., a phenomenon of so-called unbalance power capacitor voltage occurs.
Therefore, the multi-level inverter in the conventional technology lacks of a function of power capacitor voltage self-balance.